The present invention relates to a master cylinder for use in a hydraulic brake system including front and rear sets of brake cylinders.
As is well known in the art, in vehicles such as automobiles, hydraulic brake systems are widely used in which the brake pressure supply is divided into two independent circuits so that even if one of the circuits fails or malfunctions the vehicle can be safely braked by the other circuit.
In a known hydraulic brake system of this type, one of two circuits supplies a master cylinder pressure from one outlet port of a master cylinder to the front wheel brake cylinders of a vehicle while the other circuit supplies the master cylinder pressure from the other outlet port of the master cylinder to the rear wheel brake cylinders of the vehicle.
On the other hand, since the weight of a vehicle acting on the rear wheels is reduced by the so-called nose-dive phenomenon during braking, it is necessary to reduce the brake pressure supplied to the rear wheel brake cylinders to prevent locking or skidding of the rear wheels. For solving this problem, in the conventional hydraulic brake system, a brake pressure control device is disposed between the master cylinder and the rear brake cylinders which serves to reduce or limit the rear wheel brake pressure relatively to the front wheel brake pressure. However, the conventional hydraulic brake system suffers from the drawbacks that the brake pressure control device is expensive and the installation and piping of the brake pressure control device are troublesome and time consuming.
The conventional hydraulic control system also suffers from the following drawback or disadvantage: The front wheel brake cylinders connected directly to the master cylinder are fed with a brake pressure proportional to a force depressing the brake pedal of the vehicle as shown by the line a-b-d in FIG. 2 of the accompanying drawings. On the other hand, although the rear wheel brake cylinders connected to the master cylinder by way of the brake pressure control device are fed with a brake pressure shown by the line a-b in FIG. 2 which is proportional to the brake pedal depressing force when the brake pedal depressing force is below a predetermined value F.sub.1, the rear wheel brake cylinders are fed with a modulated brake pressure shown by the line b-e in FIG. 2 which is increased at a lower rate than the rate of increase in the front wheel brake pressure b-d with respect to the increase in the brake pedal depressing force upon the brake pedal being depressed with a force above the predetermined value F.sub.1. Accordingly, since the total braking force (viz. the sum of a-b-d and a-b-e) is indicated by the line a-g-h (FIG. 2) the rate at which the braking force increases with respect to the increase in the brake pedal depressing force tapers off after the modulation point (which occurs upon the brake pedal being depressed with the afore mentioned force F.sub.1). Hence to achieve a given degree of braking after the modulation point the driver must exert a disproportionately large amount of force on the brake pedal due to the modulation of the pressure fed to the rear wheels.